In-mold decoration processes involve decorating articles as they are formed, in mold, of a heated plastic material being injected into a mold cavity. Usually a tape or strip of a decorating or protective material is automatically or manually advanced, pre-fed and positioned in the mold cavity at each molding cycle, interfacing therein with the plastic material as it is filled into the mold cavity, under heat and pressure. As the article is formed, the decorating material forms on the surface of the article and becomes an integral and permanent part of the article, through thermal transfer in the in-mold decoration process. Other molding processes such as thermal forming, blow molding and compression molding or stamping may also be used for the transfer of a decorating or protective material. Sometimes the process may also be called in-mold labeling or in-mold coating, and the transferable protective material may be called a thermal transfer overcoat or durable coat layer.
The decoration tape or strip usually comprises a carrier layer, a release layer, a durable layer, an adhesive or tie-coat layer and also a layer of decorative designs (metal or ink). After the injection molding transfer, the carrier layer and the release layer are removed, leaving the durable layer as the outmost layer. The release layer allows the release of the carrier layer in a manner that minimizes damage to the display panel and enables a fully automated roll transfer process during molding. The durable layer serves as a layer to protect the decorative designs and also the molded article. To improve the conformation properties of the durable layer during injection molding of complicated three-dimensional parts, a UV curable durable layer is typically used.
U.S. Pat. No. 5,993,588 discloses a protecting layer (i.e., the durable layer) formed from a heat and radiation curable resin composition which comprises a polyfunctional isocyanate and a polymer having a (meth)acryl equivalent weight from 100 to 300 g/eq., a hydroxyl value from 20 to 500 and a weight average molecular weight from 5,000 to 50,000. The document also discloses that the durable layer may contain a UV absorber to impart weather resistance to the protecting layer on the molded article. The UV absorber also reduces the photosensitivity of the durable layer and, to some degree, improves the room light handleability of the decoration film during or before injection molding. Premature crosslinking of the durable layer in room light may occur during coating, printing, metal deposition, storage or handling of the tape or strip. Suitable UV absorbers mentioned include salicylic acid-based, benzophenone-based, diphenyl acrylate-based, benzotriazole-based, triazine-based and amine-based UV absorbers.
However, incorporating a UV absorber into a durable layer (i.e., protecting layer) formulation brings many tradeoffs. For example, it decreases the efficiency of the photoinitiator in the composition due to competitive light absorption of the UV absorber. More photoinitiators may be needed to compensate the loss of photosensitivity of the post-molding UV curing step. Sometimes, the UV absorber may also cause a yellow or fluorescence tint of the protecting layer.